Summary At approximately 1309 eastern daylight saving time (EDT), an Air Ontario de Havilland DHC-8 (ONT331) was cleared to position on runway 24 right (24R). A loss of separation occurred approximately one minute later when a Canadian Airlines Douglas DC-10 (CDN02) was cleared to land on the same runway. At approximately 1334, a second loss of separation occurred when the same controller cleared an Air Canada Airbus A320 (ACA127) to position on runway 24R and subsequently cleared a Canadian Airlines Airbus A320 (CDN962) to land on runway 24R. In both instances the landing aircraft were instructed to overshoot, and both did so safely. Ce rapport est galement disponible en franais. Other Factual Information The tower controller was working the combined controller positions of north tower and south tower. At 1308:51(1), the controller cleared ONT331 to position on runway 24R, and, at 1309:37, he transmitted control instructions to the crew. He did not include take-off clearance in this transmission. The flight crew of ONT331 apparently attempted to contact the controller to ask if they had been cleared for take-off, but at the same time, the controller was communicating with the crew of an aircraft landing on runway 24 left (24L), and the transmission from ONT331 was not heard by the controller. The transmission was not recorded. When the controller cleared CDN02 to land, he noticed that ONT331 was not departing and told the flight crew that their take-off clearance was for now. As CDN02 approached the threshold of 24R, the controller cancelled ONT331's take-off clearance and issued overshoot instructions to CDN02. At 1332, the controller cleared ACA127 to position and hold on runway 24R. About a minute later, while ACA127 was still holding, the controller cleared CDN962 to land on 24R. During the approach, the crew of CDN962 transmitted to the tower that there's an aircraft on the runway. The controller immediately issued instructions to CDN962 to pull up and go around. During the first occurrence, the shift supervisor was on his lunch break; however, there was an acting supervisor, and he noticed that ONT331 was not departing and that CDN02 was instructed to overshoot. Both the controller and the acting supervisor thought that a take-off clearance had been issued to ONT331 before CDN02 was cleared to land and that ONT331 was slow to depart. At the time of the second occurrence, the tower controller was working only the north tower position, and the supervisor was working the clearance delivery position. Staffing at the time of both occurrences was in accordance with unit policies. After clearing an aircraft to position on an active runway, the controller involved in these occurrences generally slid the flight data strip for that aircraft partially out of its flight data strip holder. Lately, however, the flight data strips had not been of a uniform width, and some slid easily in and out of the flight data strip holder, whereas others required considerable force to slide. Because of this, the controller used an alternative reminder method if the flight data strip was difficult to slide, lifting the flight data strip holder part way out of the flight data strip bay. It could not be determined what method was used for the first occurrence. At the time of the second occurrence, the controller was unable to slide the flight data strip for ACA127 because the strip was wide and would not slide easily. The controller did not use any visual cue reminder method during the second occurrence. Although the tower controller scanned the threshold of runway 24R prior to issuing the landing clearance to CDN962, he did not visually acquire ACA127. The weather during both occurrences was reported as wind 360 degrees at 5 knots, visibility 2 statute miles in rain showers and mist, and clouds based at 700 feet above ground level. The threshold of runway 24R is approximately 1.5 miles from the control tower. Factors which can affect the ability of controllers to see aircraft on the threshold include the number and type of aircraft waiting for take-off on taxiways B, H, and A and the prevailing visibility. An additional factor in this instance was the paint scheme of ACA127, which was predominantly dark. Witnesses who were in the tower during the second occurrence stated that ACA127 was quite difficult to see at the threshold of runway 24R. The control tower was equipped with monitors to display information from the airport surface detection equipment (ASDE). Controllers in the tower reported that the monitors sometimes displayed returns of non-existent aircraft, or did not display returns for aircraft which were in position on the end of runway 24R. Some of these anomalies were observed by investigators during the field phase of the investigation. Controllers stated that they checked the ASDE monitor after the second loss of separation, and there was no return for ACA127. The controller involved in the occurrences reported that he had slept only two hours during the night before the occurrences, and that his sleep pattern had been similar during several previous nights. The controller stated that he knew he was tired before he started his shift, but he assessed himself as fit to perform his duties. Significant amounts of sleep loss can have deleterious effects on one's performance, including decrements in vigilance, impairments of working memory, and increased errors in communication. People, especially those who are sleepy, often do not realize their actual level of fatigue, alertness, or performance.